The Collaborative International Dictionary of English v.0.48:

Seeker \Seek"er\, n.
   1. One who seeks; that which is used in seeking or searching.
      [1913 Webster]

   2. (Eccl.) One of a small heterogeneous sect of the 17th
      century, in Great Britain, who professed to be seeking the
      true church, ministry, and sacraments.
      [1913 Webster]

            A skeptic [is] ever seeking and never finds, like
            our new upstart sect of Seekers.      --Bullokar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

seeker
    n 1: someone making a search or inquiry; "they are seekers after
         truth" [syn: seeker, searcher, quester]
    2: a missile equipped with a device that is attracted toward
       some kind of emission (heat or light or sound or radio waves)

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "seeker":
   applicant, aspirant, bidder, candidate, chaser, claimant, delver,
   digger, follower, hopeful, hunter, perquisitor, petitioner,
   postulant, pursuant, pursuer, quester, ransacker, research worker,
   researcher, researchist, rummager, searcher, solicitant, solicitor,
   suitor, suppliant, supplicant, zetetic